Thursday, 28 March 2013

Static Timing Analysis plays major role in physical
design(PD) flow. It checks the design whether it is working properly at
specified operating frequency by checking the Timing Constraints
predefined by vendor tool are meeting by the all timing paths across design.
Here I am sharing very basic level interview questions on Static Timing
Analysis. you can find the elaborate explanations in further posts.

1. What is Static Timing Analysis(STA)?

A. Static timing analysis is a method for
determining if a circuit meetstiming constraintswithout having to simulate. So, it validates the design for desired
frequency of operation, without checking the functionality of the design.

2. What is Setup Time?

A. Setup time is the amount of time before
the clock edge that the input signal needs to stable to guarantee it is
properly accepted on the clockedge.

3. What is Holdtime?

A. Hold time is the amount of time after
the clock edge that the input should be stable to guarantee it is
properly accepted on the clock edge.

4. what is setup and Hold time violations?

A. Voilating above setup and hold time
requirements is called setup and hold time violations. If there is setup and
hold time violations in the design does not meet the timing requirements and
the functionality of the design is not reliable. STA checks this setup
and hold violations.

5. How can you avoid setup time violations?

A. 1. Play with clock (Useful) skew.

2. Redesign
the flipflops to get lesser setup time

3. The
combo logic between flipflops should be optimized to get minimum delay

4. Tweak launch
flip-flop to have better slew at the clock pin, this will make launch flip-flop
to be fast there by helping fixing setup violations.

6. How can you avoid hold time violations?

A. 1. By adding delays using buffers

2. By adding
lockup-latches

7. What is Slack?

A. The difference between Required
Arrival Time and Actual Arrival Time is called as Slack. The amount
of time by which a violation (Either setup or Hold) is avoided is called the
slack.

8. What is Negative slack?

A. The difference between required
arrival time and actual arrival time is Negative, then it is called as
Negative slack. If there is
negative slack, the design is not meeting the timing requirements and the paths
which have negative slack called as violating paths.
We have to fix these violations to make the design
meeting timing.

9. What is Positive slack ?

A.The difference between required arrival
time and actual arrival time is positive, then is called as positive
slack. If there is positive slack, The design
is meeting the timing requirements and still it can be
improved.

10. In back-end design which violation has more priority? Why? A. In back-end design, Hold violation has more priority than Setup Violation. Because hold violation is related to data path and not depends on clock. Setup violation can be eliminated by slowing down the clock (Increasing time period of the clock).